(a) Field of the Invention
The present invention relates to an amplifier for use to amplify electrical signals with a power-supply circuit providing a stabilized power-supply current.
(b) Description of the prior art
An amplifier generally comprises a signal amplifying circuit for amplifying an input signal to the amplifier and a power-supply circuit for supplying power to the amplifying circuit for operation. In general, the overall signal-amplifying operation of the amplifier considerably depends upon the performance of the power-supply circuit, such as voltage regulation and impedance, including power supply lines through which power is supplied to the signal-amplifying circuit. If the voltage regulation of the power-supply circuit is poor, the power-supply voltage applied to the signal-amplifying circuit will fluctuate due to changes in the power-supply current for the circuit in accordance with the input signal, and in turn, the signal-amplifying operation of the signal-amplifying circuit will become deteriorated. In the instance wherein the signal-amplifying circuit has a plurality of signal channels, such fluctuations in the power-supply voltage can lead to a poor separation between the signal channels. Thus, a power-supply circuit employed in an amplifier has to be carefully designed to provide as good performance as possible. However, it generally is a very difficult and costly task to achieve such an ideal power-supply circuit that has no impedance and zero voltage fluctuation.
In the past, therefore, a larger part of the amplifier-designing effort has been paid to improvement of the SVRR (Supply voltage rejection ratio) of the signal-amplifying circuit in an amplifier, rather than the power-supply circuit, by means of the employment of a cascode configuration, stabilization of biasing voltages for amplifying elements in the signal-amplifying circuit, NFB (negative feedback) technique, and like means. Such approaches, however, are liable to make the entire amplifier more complicated and expensive, and practically are not able to sufficiently overcome the problems due to the limited performance of the employed power-supply circuit.
An amplifier usually has another problem which is caused by signal current flow on the ground line in the amplifier, which line may be one of the power supply lines. That is, various signal currents, main part of which is the output signal current of the signal-amplifying circuit, flow on the ground line and develop corresponding voltage drops along the line. These voltage drops can be fed back to the input side of the signal-amplifying circuit, rendering the operation of the amplifier unstable, since such voltage drops change with the input signal which is applied to the amplifier. In case the amplifier has a plurality of signal channels, the above feed-back of the ground-line voltage drops can severely hamper the channel-separation characteristic. The improvement of the SVRR of the signal-amplifying circuit is not much effective for overcoming this problem.